1. Field of the Invention
The present invention relates to a process for producing highly refined methacrylic acid containing little dibasic acids such as maleic acid and citraconic acid efficiently on an industrial scale from an aqueous methacrylic acid solution obtained by cooling a methacrylic acid-containing reaction gas formed by oxidizing methacrolein, isobutyl aldehyde, t-butanol or isobutylene catalytically in a vapor phase.
2. Description of the Related Art
Methacrylic acid is generally refined by extracting it from an aqueous methacrylic acid solution, which is obtained by cooling and condensing a methacrylic acid-containing gas formed by oxidizing a 4 C compound such as methacrolein, isobutyl aldehyde, t-butanol or isobutylene catalytically in a vapor phase, with an organic solvent, which does not mix with water and hence forms separated phases with the condensed aqueous solution, followed by distillation of the extract. The organic solvent used for this purpose may include, for example, aliphatic hydrocarbons having 5 to 9 carbon atoms, aromatic hydrocarbons such as benzene and tolunene and esters such as methyl methacrylate.
The process for producing methacrylic acid by the catalytic oxidation of the above-described 4 C compounds in one or two reaction stages or more is now actually practiced on an industrial scale. However, a variety of side reactions occur simultaneously in the catalytic oxidation reaction and therefore the reaction product gas contains a variety of by-produced impurities in addition to the desired methacrylic acid. The miscellaneous by-produced impurities get mixed partially in the aqueous methacrylic acid solution obtained by cooling and condensing the reaction product gas and in the extract from the aforesaid aqueous solution with the organic solvent. The principal by-produced impurities include acetaldehyde, acetone, acetic acid, acrolein, methacrolein, acrylic acid, maleic acid, citraconic acid, aromatic aldehydes, aromatic carboxylic acids, heterocyclic compounds and the like.
Among these by-produced impurities, acetaldehyde, acetone, acetic acid, acrolein and methacrolein, which are lower boiling point components than methacrylic acid, can be separated by distillation, and aromatic compounds, higher boiling point components, can also be separated by distillation. However, it is difficult to separate by distillation dibasic acids such as maleic acid and citraconic acid transferred to the extract phase along with methacrylic acid during the extraction.
With a view to removing these hardly separable impurities from methacrylic acid, the present inventors have examined a process in which the impurities-containing methacrylic acid extract prior to the distillation is brought into contact with a basic ion-exchange resin, followed by refining of the resultant extract by distillation. Thus, inventions of U.S. Pat. No. 4,879,412 and Japanese Patent Laid-Open No. 193740/1989 have been proposed. The present inventors have also studied a process in which the impurities-containing methacrylic acid extract prior to the distillation is brought into contact with a basic amine, followed by refining of the resultant extract by distillation. As a result, an invention of U.S. Pat. No. 5,196,578 has been proposed.
According to the processes proposed by the inventors, it has been proved that impurities including dibasic acids such as maleic acid contained in methacrylic acid are adsorbed or made high boiling point components by bringing the methacrylic acid extract into contact with a basic ion-exchange resin or a basic amine, and the extract thus treated is subjected to the subsequent distillation step to produce methacrylic acid of a high quality.
To solve the aforementioned problems, in processes other than those proposed by the present inventors, for example, in the specification of Japanese Patent Laid-Open No. 152438/1985, there is proposed a process comprising the steps of subjecting an aqueous methacrylic acid solution obtained by cooling and condensing the reaction product gas to an evaporation step such as distillation to remove low boiling point components, extracting methacrylic acid from the resulting solution with an organic solvent that forms an azeotropic composition with water, such as an aromatic hydrocarbon, bringing the extract into contact with an aqueous hydrogensulfite solution to separate the mixture thus obtained into two phases, and subjecting only the oily phase to a distillation and refining step. Further, for example, in the specification of Japanese Patent Laid-Open No. 178842/1985, there is proposed a process comprising the steps of cooling and condensing the reaction product gas to an aqueous methacrylic acid solution, extracting methacrylic acid from the aqueous solution with an aromatic or aliphatic hydrocarbon solvent, and adding an alkali or an aqueous hydrazine solution to the extract to separate maleic acid and the like as a salt by precipitation.
In these conventional processes, however, substances, which do not originally exist in their process solutions, are incorporated so that waste water and wastes, which are unrecoverable to and unreusable in the processes, are evolved therein to render the processes defective. Therefore, expenses are necessary to treat the waste water and wastes that are unrecoverable to and unreusable in the processes. Consequently, the processes involve an essential problem of deteriorated economical efficiency.
Further, a reaction of polymerizing methacrylic acid takes place due to the substances incorporated, giving rise to the problem of equipment being contaminated.